First ultrastructural description of an apomictic opsiblastic egg in freshwater Gastrotricha
First ultrastructural description of an apomictic opsiblastic egg in freshwater Gastrotricha
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DOI：
10.1002/jmor.21659
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发表时间：
2023-11
影响因子：
1.5
通讯作者：
T. Q. Araújo;Simon King‐Trudeau;Joanna VanDyke;R. Hochberg
中科院分区：
文献类型：
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作者：
T. Q. Araújo;Simon King‐Trudeau;Joanna VanDyke;R. Hochberg
淡水腹毛具有两个生命周期，据说包括产生三种类型的卵：无融合生殖和快速孵化(速生卵子)，无融合生殖和延迟孵化(光学卵子)，以及带有斑块的卵(可能来自混合卵子)。虽然关于卵子发生和蛋壳结构的一些细节已经为人所知，但关于其他卵子类型的细节却很少。在这里，我们首次提供了淡水鳞毛虫产卵的顶生卵的超微结构描述。扫描电子显微镜显示，蛋壳表面装饰着长长的扁平的柱状结构，中心位于多边形板的中心，这些多边形板沿其外围凹陷。透射电子显微镜显示，这些凹坑导致巨大的管状空间迷宫，并在贝壳厚厚的顶层形成更大的空洞。卵壳的基底层是无定形的，并连接到一个穿过卵母细胞外空间的细小纤维网络，并在未分裂的卵母细胞周围形成一层保护膜。未分裂的卵母细胞有一层致密的外周卵质，包围着细胞器的核心，包括线粒体、膜结合的分泌颗粒、内质网，以及富含核糖体的颗粒状细胞质中的单个核。分泌颗粒是最丰富的细胞器，推测含有富含脂肪的蛋黄，这些蛋黄将被用作延迟卵裂的能量，从而起到暂时分散的作用。将这些数据与无脊椎动物休眠卵的精细结构在系统发育谱上进行比较，以确定鳞毛虫视母细胞卵结构的新颖性。
Freshwater gastrotrichs have a biphasic lifecycle that reputedly involves the production of three types of eggs: apomictic and fast hatching (tachyblastic ova), apomictic and delayed hatching (opsiblastic ova), and plaque‐bearing eggs (potentially derived from mixis). While some details of oogenesis and eggshell structure are known for tachyblastic ova, there are few details on other egg types. Here, we provide the first ultrastructural description of the oviposited opsiblastic eggs of the freshwater gastrotrich, Lepidodermella squamata. Scanning electron microscopy revealed the eggshell surface to be ornamented with long flattened pillar‐like structures centered on polygonal plates that are pitted along their periphery. Transmission electron microscopy showed the pits to lead to a vast labyrinth of tubular spaces and larger cavities throughout the thick apical layer of the shell. The basal layer of the shell is amorphous and connected to a network of fine fibers that traverse an extra‐oocyte space and forms a protective sheet around the uncleaved oocyte. The uncleaved oocyte has a dense layer of peripheral ooplasm surrounding a core of organelles including mitochondria, membrane‐bound secretion granules, endoplasmic reticulum, and a single nucleus in a granular, ribosome‐rich cytoplasm. Secretion granules are the most abundant organelles and presumably contain lipid‐rich yolk that will be used as energy for delayed cleavage, thus functioning in temporal dispersal. These data are compared to the fine structure of invertebrate resting eggs across the phylogenetic spectrum to determine the novelty of opsiblastic egg structure in L. squamata.